Liquid coating composition



United States Patent 3,118,048 LllQUiD @GAllllNG CUMPOSETION .l'ohn lit. Fisher, .ha, and Ira W. Dance, Dayton, Ohio, assignors to Industrial Metal Frotectives, llnc., Dayton, @hio, acorporation of Delaware No Drawing. Filed July is, H61, Ser. No. 124,796 9 Claims. {CL M l-92) This invention relates to liquid coating compositions for providing corrosion resistant coatings on metal parts and the like, and more particularly to an improved metallically pigmented, electrically conductive liquid coating composition for providing an improved anticorrosive coat ing on metal parts so that such parts, when coated, may be welded together.

Corrosion of a metallic substrate is primarily an electrochemical phenomena wherein the metallic component of the substrate undergoes an oxidation reaction involving the loss of electrons and a chan e in a valence. The ease with which a particular metal or alloy corrodes is a function of an electrochemical activity which is conveniently expressed as an oxidation potential relative to the hydrogen-hydrogen ion couple. Protection against corrosion may be achieved by placing in intimate contact with the base a metal having an oxidation potential greater than the base so as to provide a source which is oxidized more readily than the base. The usual arrangement involves positioning such a metal of higher oxidation potential in close contact with the base, as by coating and the like, so as to provide a metal on that surface of the base which is capable of providing a readily oxidizable metal operating to protect the base. A Well known example of a protective coating of this type is a liquid coating composition including a finely divided metallic pigment, such as zinc, dispersed in a vehicle of a film forming binder which is employed to cause the pigment to adhere to a suitable substrate, for example ferrous alloys, such as steel. Iron has an oxidation potential of about +0.44 volt while zinc, for example, has an oxidation potential of about +0163 volt, and thus it can be seen that the zinc which is in intimate contact with the iron will operate galvanically to protect the iron from corrosion by giving off electrons, i.e., become oxidized before oxidation of the iron substrate.

Many of the metals hi h in the electromotive series are too active to be employed per se in corrosion resistant coatings of the type above described, and these metals include, for example, cesium, sodium and the like. It has been discovered in accordance with the present invention that alloys of the metals high in the electromotive series may be employed in relatively small amounts to improve the overall corrosion protection of a liquid coating composition.

Accordingly, it is a primary object of the present invention to provide a novel liquid coating composition which is capable of providing increased corrosion protection when applied to a substrate and dried thereon, and wherein the protection afforded by the dried coating ex tends for relatively long periods of time.

Another object of the present invention is the provision or" a liquid coating composition of the type described providing superior corrosion resistant properties for an extended period of time and which when dried on a suitable substrate provide for Weldability thereof.

Still another object of the present invention is the provision of a liquid coating composition including a component capable of acting as a thixotropic agent and which does not materially interfere with the weldability or corrosion protection of the dried coating.

A further object of the present invention is the provision of a liquid coating composition for application to a metal substrate for providing, when dried thereon, a Weld- 2 able and corrosion resistant coating, and wherein the liquid composition includes a finely divided metallic dust having an oxidation potential greater than the substrate and a finely divided alloy containing at least one metal having an oxidation potential greater than that of the metallic dust.

Other objects and advantages of the invention will be apparent from the following description and the appended claims.

Insofar as corrosion and galvanic protection are concerned, one of the prime considerations involves the relationship between the metallic pigment and the metal constitutents of the substrate. Considered from the viewpoint of released electrons, the alkali metals appear to be most advantageous, however, the action of such metals, because of their high reactivity, is far too fast to provide a protective coating capable of imparting galvanic or cathodic protection for any extended period of time. As a practical matter, materials such as magnesium, alumium, titanium, zinc, iron, cadmium, cobalt, nickel, molybdenum, and copper represent materials which may be employed, provided the oxidation potential of the substrate is less than that of the particular material.

It is preferred that the metal in the coating be in a finely divided state for ease of distribution in the film forming resin and for uniformity of application to the substrate. It has been found in accordance with the present invention that particle sizes in excess of about microns may be too large to provide an adherent coating capable of providing corrosion and galvanic protection for extended periods of time. With particle sizes in excess of about 150 microns, oxidation of the pigment particle during the course of its sacrificial action results in the creation of sizable voids in the coating, and ultimately the coating may peel and crack from the surface. It has been found in accordance with the present invention that advantageous results have been produced with particle sizes whose average ranges from about two microns up to about sixty microns. In instances where it is desired to provide corrosion and galvanic protection as well as Weld-ability, it is preferred to employ the larger sized particles for reasons which will be discussed more fully hereinbelow.

The finely divided metallic component of the protective coating is dispersed in a resin capable of adhering to the metallic base or substrate for maintaining the finely divided metal on the base, and generally operates as a carrier for the metallic component. The resin also operates, to some extent, to provide mechanical and physical protection of the base by virtue of the film which is formed as the resin solidifies.

Resins which have been employed and produced satisfactory results in accordance with the present invention include phenolic modified and maleic modified alkyd resins, the glycerol isophthalate group of resins, phenolic or maleic modifications thereof, the epoxy resins, glyptal resins, ester gums and damrnar gums, polyester resins, polybutene resins, polyurethanes, and hydrocarbon resins such as methylated paraffinic hydrocarbons, polydiene, polyturpene, polyethylene and polypropylene resins, or mixtures of the above resins. In the case of alkyds, it is preferred that the alkyd have an acid number below about ten and preferably about five. Within this group of alkyds may be included linseed modified alkyds, soya modified alkyds, mixtures of bodied linseed oil and alkyds, or any of the well known oil or acid modified alkyds having an acid number as previously mentioned.

it is noted that each of the resins previously mentioned in the dried state exhibit a characteristic resistance, that is, the resistance of the film formed by each of the resins varies from one resin to the next. Because the protective coating in accordance with the present invention may be salaries welded and is employed to provide galvanic protection, it is desirable to control the amount of conductive material in the dried coating in order to provide a protective coating having the desired electrical characteristics insofar as weldability and galvanic protection are concerne Satisfactory results have been achieved in accordance with the present invention by employing a composition wherein the metallic pigment constitutes approximately 76% to 97% of the total solids of the composition in order to provide adequate galvanic action or cathodic protection of the coated part. in the case of resins having a high dielectric constant, i.e., those having a high resistance, such as polyurethane and polyester resins, it is preferred to employ about 90% to 97% conductive solids in the composition, while in the case of epoxy type resins the use of between 80% to 90% conductive solids operates in a satisfactory manner. With resins of the alkyd type, previously mentioned, approximately 75% to 85% conductive solids in the composition produce acceptable results.

A convenient method of determining the overall efiiciency of the film constants in testing the electrical resistance of the coating as described in detail in the military specification MlL-P26915A (USAF). All of the above percentages operate satisfactorily to provide a coating having an electrical resistance which is less than the ohms per square inch thickness as set forth in the above identified military specification.

Additionally, the welding properties of the dried coating appear to be greatly influenced by the ratio of resin or hinder solids to the amount of conductive pigments present, and satisfactory results have been obtained with compositions wherein the ratio of resin to pigment in the composition falls within the range from a low of 12 parts resin to or parts pigment to a high of about 4 parts resin to parts pigment, that is, with the pigment present in the amount of 5 to 20 times the amount of resin. With a composition including polybutane type resins, the resin to pigment ratio may be as low as 1:1.

At ratios above those previously mentioned, that is, with the resin present in excess of 1:5, the coating possesses an electrical resistance which is too high for efficient welding and may be too high to provide efiicient galvanic and cathodic protection. On the other hand, with too high a ratio of pigment, for example in excess of 1:20, the coating contains insufficient resin to provide a mechanically stable coating. Additionally, with such relatively large proportions of pigment, the gradual consummation of pigment during the course of protection creates voids, and the coating becomes dimensionally unstable exhibiting cracks and blisters after continued ex posure to the elements and particularly salt sprays, for example.

As was mentioned previously, the particle size of the pigment is of importance because of the dimensional stability of the coating as Well as the durability of the coating. From the standpoint of durability, and for a given resin to pigment ratio, a finer particle size is the desired condition since the particles of pigment are small and as they are consumed during the course of their action the mechanical strength of the coating is not materially reduced. In contrast to this, it has been found, for a given resin to pigment ratio, that larger particles operate more satisfactorily from the standpoint of weldability since the larger particles provide electrically conductive paths through the coating in which the paths have less electrical resistance than is the case with relatively small particles. This situation is analogous to a comparison between a relatively fine conductive wire and a relatively thick conductive wire wherein both wires are of the same material. As was mentioned previously, it is preferred that the particle size be in the range of about two microns to about seventy-five, or that the pigment have an average size falling within the above range.

One feature of the present invention relates to the use of a controlled amount of finely divided alloy vhich operates to increase significantly the duration of the corrosion protection of a metallically pigmented coating composition over that achieved with coating compositions wherein the alloy is not present. It has been found in accordance with the present invention that the addition to the composition of about 0.5% to about 6%, and preferably about 1% to 3% of a finely divided alloy operates to increase the duration of protection by a factor of about l0. The metallic pigment portion of the coating composition selected from materials having an oxidation potential higher than that of the base metal material, for example, in the case of ferrous bases, cadmium, zinc, manganese, aluminum and magnesium may be employed. As a practical matter many compositions include Zinc as the metallic pigment since zinc operates satisfactorily with most of the base materials, especially the ferrous base materials.

It has been discovered in accordance with the present invention that alloys containing at least one metal higher in the electromotive series than the metallic pigment operate to prolong the effective corrosion resistance of the protective coating. While the exact mechanics of such improved results are not. completely understood at the present time, it may be postulated that the alloys, for example, a magnesium-aluminum alloy, operate to effect a controlled release of electrons during the corrosion process thus providing enhanced protection of the base metal. Since the alloy includes at least one metal capable of releasing electrons at a rate faster than that of the sacrifical metallic pigment, the combined action of the alloy and the sacrificial pigment, in fact, operates to extend the life of the coating.

As will be understood, the presence of the alloy mate rials with the metallic pigment does not appreciably alter the weldability of the coating. As a matter of fact there appears to be a slight improvement in the weldability since about 0.5% to 6% by weight of an alloy material in finely divided form, as previously noted, operates to de crease the electrical resistance of tie coating thereby effecting a reduction in the current required for weldmg.

In the case of liquid vehicles including a solvent in a synthetic resin component of the type previously mentioned, there may be present in the solvent and/ or film forming resin certain components which react with the finely divided pigment, which reaction may use up or waste a substantial portion of the pigment or nullify the utility thereof for the desired corrosion resistance, cathodic protection, and electrical conductivity particularly in the event that the coated parts are thereafter to be welded or spot: welded. The several dis es resulting from a reaction between the metallic pigment and the reactive components of the liquid vehicle are disclosed in copendlng application Serial No. 779,541, filed December 11, 8 and assigned to the same assignee.

In order to provide a ready mixed composition which may be stored without rupture of the container because of hydrogen evolution during the reaction between the components of the liquid vehicle and the metallic pigment, it is preferred to employ materials capable of effectively neutralizing the reactive components of the vehicle prior to adding the pigment thereto. To this end, a moisture absorption component or desiccant is added to the vehicle along with a basic material capable of reacting with the acids in the vehicle prior to addition of the pigment. Satisfactory results in rendering any reactive components of the vehicle inactive have been produced by the addition of plaster of Paris, calcium oxide in the form of quick lime, anhydrous silicate gel, potassium hydroxide powder, or other suitable materials. in addition to the above desiccant and acid neutralizing agents, thixotropic agents may be added for adjusting and improving the final rheology of the composition. Components such as castor oil, soap, Thixcin are well known thixotropic agents or thickeners employed alone or in combination with such materials as lead, cobalt, manganese and zirconium naphthenates, the latter materials being employed as driers.

In addition to the preceding thixotropic agents, extremely satisfactory results have been produced by employing agents available commercially under the trademark Ciroxin, which is believed to be a hydrogenated castor oil. As is well known, castor oil is 70 to 95% triricinolein, that is, the glyceride of ricinoleic acid. A hydrogenated product operates quite satisfactorily to maintain the metallic portion of the composition uniformly and evenly distributed therethrough in order to effect a substantial increase in the shelf life of the premixed or ready mixed liquid coating composition. The advantage of using Ciroxin stems from the fact that the Ciroxin is easily added to the mixture during formulation thereof by employing heat. In contrast to this, many of the other thixotropic agents heretofore employed require a ball millin operation which consumes considerable time in formulation techniques. Additionally, the Ciroxin does not interfere materially with the charcteristics of the final coating, and in fact, improves the welda'bility by providing a more uniform dispersion of pigment throughout the dried coating.

Merely as illustrative of compositions embodying and for practicing this invention, the following may be noted:

Hydrocarbon resin (available under the trademark Piccopal) 18.6 Solvent 124.6 Thixotropic agent (Ciroxin) 7.9 Epoxy ester resin (50% solution in xylene) 309.5 Anti-gassing agent (CaO) 3.0 Desiccant (silica gel) 3.0 Aluminum 6.0 M /Al Alloy 5050 alloy .s 24.5 Solvent 92.9 Powdered Metallic Pigment (Zinc) 13940 Hydrocarbon resin (available under trademark Piccopal) 14.2 Solvent 94.9 Thixotropic agent (Ciroxin) 6.0 Epoxy ester resin (50% solution in xylene) 235.6 Anti-gassing agent (CaO) 2.2 Desiccant (silica gel) 2.2 Aluminum 4.5 Mg/Al alloy 60-40 alloy 18.7 Solvent 230.7 Powdered metallic pigment (zinc) 106 1.0

III.

Phenolic modified resin (50% solution) 360.0 Solvent 120.0 Thixotropic agent (Ciroxin) 7.0 Anti-gassing agent (CaO) 3.0 Desiccant (silica gel) 3.0 Aluminum 5.0

Alloy 20.0 Pigment 1400.0

Maleic modified alkyd (50% solution in xylene) 400.0 Solvent 194.0 Lead Napthenate (drier) 2.0 Cobalt Napthenate (drier) 2.0 Thixotropic agent (Thixcin) 3.0 A1 pigment 300.0 Zinc pigment 700.0 Alloy Mg-Al alloy (65-35) 45.0

Glycerol Isophthalate resin (50% solution) 308.0 Al Paste 36.0

Solvent 149.0 Lead Napthenate 3.0 Cobalt Napthenate 1.0 Desiccant 1.0 Anti-gassing agent 1.0 Alloy 14.0 Thixotropic agent 15.0 Pigment 1000.0

Polyurethane 40.0 Solvent 40.0 Thixotropic agent 3.0 Alloy 44.0 Pigment 800.0

In the case of a base member formed of a nickel, finely divided iron may be used as the pigment portion of the composition and an alloy of copper and magnesium or molybdenum iron may be emphasized to enhance or increase the duration of the corrosion protection. For ferrous base materials, finely divided chromium may be employed as the pigment portion with about 0.5% to about 6% of magnesium-nickel alloy. It is preferred however to employ the combination of zinc with a magnesiumaluminum alloy since these materials are readily available commercially and operate to provide satisfactory results with most base materials.

The above specific formulations employing Ciroxin were prepared by adding the Ciroxin to the solvent and heating the resultant mixture to about to 280 F. at which point the Ciroxin goes into solution with the solvent. Thereafter the desiccant, the anti-gassing agents, and the resinous film forming components are added, while mixing and maintaining the solution at a temperature of 120 F. As the solution cools down, it takes on a cloudy appearance. Thereafter the metallic pigment and finely divided alloys are added and the mixture is again stirred to provide a uniform dispersion.

It is to be noted that by adding the desiccant and the anti-gasing agent to the vehicle prior to addition of the metallic pigment, the reactive components of the vehicle are neutralized or otherwise rendered inactive prior to the addition of the metallic pigment. For this reason, there is very little, if any, pigment present in the form of nonconductive pigment, and essentially all of the pigment present in the composition is unreacted and available as an active component of the final coating. Stated another way, the entire amount of pigment present in the composition is present in the form of electroconductivc pigment as opposed to salts thereof.

One feature of the present invention which is of interest is the healing action that takes place on the exposed area of the base metal attributable to the deposition of salts on the exposed area. This feature is of considerable interest with respect to corrosion protection extending over long periods of time. It has been found that with proper resin to pigment ratios, the consumption of the active components of the protective coating operate to deposit or otherwise form a metal salt or an insoluble zinc hydroxide on those portions of the base member which have been exposed. The metal salts are relatively inert to further reaction under ordinary conditions and operate to form an inert coating on those portions of the base material which may have been exposed in order to provide protection against corrosion even though the film may have been chipped away or otherwise removed from that particular portion of the surface. The healing action above described, is a characteristic of a properly formulated paint composition, and each of the compositions specifically set forth above operates to provide the above healing action.

If the coated parts are to be welded, at least one surface and preferably both surfaces of each article are coated with one of the compositions previously described, and the coating is allowed to dry. Thereafter, the articles are placed in contact such that the areas to be spot welded are overlapping and a welding current is passed through the articles to fuse the articles together in the area of each spot weld. Passage of a welding current through the articles and the coating therebetween operates to effect a fusion of the metallic pigment of the coating in the area of the weld.

The improved compositions above described for providing cathodic or galvanic protection as well as providing a weldable surface operate quite satisfactorily due to the proper relationship of resin to pigment ratio and percentage of metallic solids to total solids as previously described. Additionally, selecting the size of the pigment as above described operates to provide a coating which may be welded satisfactorily.

Enhanced or improved corrosion protection has been achieved by the addition of an alloy which operates to effect a controlled release of electrons during the course of sacrificial action of the components of the composition. It is also possible in accordance with this invention to provide ready mixed or premixed compositions which may be stored without rupture of the container, and wherein the shelf life thereof has been extended by provision of suitable thixotropic agents which operate to disperse the metallic components of the composition evenly and uniformly therethrough.

While the processes and products herein described constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to these precise processes and products, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. A liquid coating composition for application to a metal substrate having a given oxidation potential for providing, when dried on said substrate, a corrosion resistant and electroconductive weldable coating thereon, comprising a finely divided solid inorganic water-insoluble metallic electroconductive pigment having an oxidation potential greater than the oxidation potential of said substrate, said pigment being uniformly dispersed in a liquid binder and carrier therefor including a solvent and a synthetic film forming resinous component for ad.- hering said pigment to said substrate, a finely divided metallic alloy dispersed in said coating composition including a first metal having oxidation potential greater than that of said pigment and a. second metal having an oxidation potential less than that of said pigment, said sec ond metal acting as an agent effecting controlled release of electrons from said first metal for increasing the corrosion protection provided by said pigment, said electroconductive pigment being present in an amount of 75% to 97% of the total solids in the dried coating, and the ratio of resin to pigment in said liquid coating composition falling approximately within the range of 1:1 to 1:20.

2. The composition as set forth in claim 1 wherein the metallic alloy is present in an amount of approximately 0.5% to 6% by weight of the liquid composition.

3. A liquid coating composition for application to a metal substrate having a given oxidation potential for providing, when dried on said substrate, a corrosion resistant and electroconductive weldable coating thereon, comprising a finely divided solid inorganic water insoluble metallic electroconductive pigment having an oxidation potential greater than the oxidation potential of said substrate, said pigment being uniformly dispersed in a liquid binder and carrier therefor including a solvent and a synthetic film forming resinous component for adhering said pigment to said substrate, said film forming component being selected from the class consisting of phenolic, maleic and linse d oil modified alkyd resins, glycerol isophthalate resins and phenolic and maleic modifications thereof, epoxy resins, glyptal resins, ester and dammar gum resins, polybutene resins, polyurethane resins, methylated parafinic hydrocarbon resins, polydiene, polyturpene, polyethylene and polypropylene resins, and compatible mixtures thereof, a finely divided metallic alloy dispersed in said coating composition including a first ietal having oxidation potential greater than that of said pigment and a second metal having an oxidation potential less than that of said pigment, said second metal acting as an agent effecting controlled release of electrons from said first metal for increasing the corrosion protection provided by said pigment, said electroconductive pigment being present in an amount of 75% to 97% of the total solids in the dried coating, and the ratio of resin to pignent in said liquid coating composition falling approximately within the range of 1:1 to 1:20.

4. A ready mixed liquid coating composition for plication to a metal substrate having a given oxidation potential for providing, when dried on said substrate, a corrosion resistant and electroconductive weldable coating thereon, comprising a finely divided solid inorganic water insoluble metallic electroconductive pigment having an oxi ation potential greater than the oxidation potential of said substrate, said pigment being un 'ornily dispersed in a liquid vehicle system therefor including a solvent and a synthetic film formin resinous component and driers therefor for adhering said pigment to said substrate, said vehicle having reactive components susceptible to chemical reaction and combination with said pigment, a finely divided metallic alloy dispersed in said coating composition including a first metal having oxidation potential greater than that of said pigment and a second metal having an oxidation potential less than it of said pigment, said second metal acting as an agent effecting controlled release of electrons from said first metal for increasing the corrosion protection provided by said pigment, said electroconductive pigment being present in an amount of 75% to 97% of the total solids in the dried coating, the ratio of resin to pigment in said liquid coating composition falling approximately within the range of 1:1 to 1:20, and said pigment being present in the liquid composition as the elemental metal and uncombined and unreacted with other components of said composition.

5. A liquid coating composition for application to a metal substrate having a given oxidation potential for providing, when dried on said substrate, a corrosion rcsistant and electroconductive weldable coating thereon, comprising a finely divided solid inorganic water insolule metallic electroconductive pigment having an oxidation potential greater than the oxidation potential of said substrate, said pigment being uniformly dispersed in a liquid binder and carrier therefor including a solvent and a synthetic film forming a resinous component for adhering said pigment to said substrate, a finely divided metallic alloy present in said coating composition in an amount of 0.5% to 6% by weight and including a first metal having an oxidation potential greater than that of said pigment and a second metal having an oxidation potential less than that of said pigment, said second metal acting as an agent efecting controlled release of electrons from said first metal for increasing the corrosion protection provided by said pigment, said electroconductive pigment being present in an amount of 75% to 97% of the total solids in the dried coating, the ratio of resin to pigment in said liquid coating composition falling approximately within the range of 1:1 to 1:26, and said pigment having an average particle size falling in the range of two to sixty microns.

6. A ready mixed liquid coating composition for application to a metal substrate having a given oxidation potential for providing, when dried on said substrate, a corrosion resistant and electroconductive weldable coating thereon, comp ing a finely divided solid inorganic water insoluble metallic electroconductive pigment having an oxidation potential greater than the oxidation potential of said substrate, said pigment being uniformly dispersed in a liquid vehicle system therefor including a solvent and a synthetic film forming a resinous component for adhering said pigment to said substrate, said vehicle having reactive components susceptible to chemi cal reaction and combination with said pigment, a finely divided metallic alloy present in said coating composition in an amount between 0.5% and 6% by weight and including a first metal having oxidation potential greater than that of said pigment and a second metal having an oxidation potential less than that of said pigment, said second metal acting as an agent effecting controlled release of electrons from said first metal for increasing the corrosion protection provided by said pigment, said electroconductive pigment being present in an amount of 75% to 97% of the total solids in the dried coating, the ratio of resin to pigment in said liquid coating composition falling approximately within the range of 1:1 to 1:20, said pigment being present in the liquid composition as the elemental metal and uncombined and unreacted with the other components of said composition, and said pigment having an average particle size falling in the range of two to sixty microns.

7. The method of spot welding metallic articles which comprises applying to a surface of at least one of the articles to be welded together at least one coating of a liquid coating composition for providing, when dried upon the articles, a continuous surface coating thereover effecting corrosion resistance, weldable electroconductivity and cathodic protection, said liquid coating composition including a finely divided solid inorganic waterinsoluble metallic electroconductive pigment having an oxidation potential higher than that of the metal of said metallic article for providing said conductivity and cathodic protection, said pigment being uniformly dispersed in a liquid binder and carrier therefor including a solvent and a synthetic water-insoluble film forming resin component for providing said continuous film for corrosion resistance, a finely divided alloy material dispersed in said composition including a first metal having an oxidation potential greater than that of said pigment and a second metal having an oxidation potential less than that of said pigment, said alloy operating to increase the electrical conductivity of said protective coating during welding and increasing the corrosion protection provided by said electroconductive pigment, the ratio of said resin to said pigment in the liquid coating composition falling approximately within the range of 1:1 to 1:20, said pigment being present in an amount of 75% to 97% of the total solids in the dried coating, contacting at least a portion of said one article with another article of metal to be welded, passing a welding current through said articles of metal and said coating thereon at the spots selected for welding, and fusing said articles together in the area of each said spot weld.

8. The method of spot welding metallic articles which comprises applying to at least one surface of said articles to be welded together at least one coating of a liquid coating composition for providing, when dried upon the articles, a continuous surface coating thereover effecting corrosion resistance, weldable electroconductivity and cathodic protection, said liquid coating composition including a finely divided solid inorganic water-insoluble metallic electroconductive pigment having an oxidation potential higher than that of the metal of said metallic article for providing said conductivity and cathodic protection, said pigment being uniformly dispersed in a liquid binder and carrier therefor including a solvent and a synthetic water insoluble film forming resin component for providing said continuous film for corrosion resistance, a finely divided alloy material dispersed in said compositions including a first metal having an oxidation potential greater than that of said pigment and a second metal having an oxidation potential less than that of said pigment, said alloy being present in an amount of about 0.5% to 6% by weight of said liquid composition, said pigment being present in an amount of about to 97% of the total solids in the dried coating, said alloy operating to increase the electrical conductivity of said protective coating during welding and increasing the corrosion protection provided by said electroconductive pigment, the ratio of said resin to said pigment in the liquid coating composition falling approximately within the range of 1:1 to 1:20, placing at least a portion of the coated surface of one article in contact with at least a portion of the surface of a second article, passing a welding current through the two articles of metal and the coating between them at the spots selected for welding, and fusing said articles and the coating between them together in the area of each spot weld.

9. The method of spot welding metallic articles which comprises applying to opposite surfaces of the articles to be welded together at least one coating of a liquid coating composition for providing, when dried upon the articles, a continuous surface coating thereover effecting corrosion resistance, weldable electroconductivity and cathodic protection, said liquid coating composition including a finely divided solid inorganic water-insoluble metallic electroconductive pigment having an oxidation potential higher than that of the metal of said metallic article for providing said conductivity and cathodic protection, said pigment being uniformly dispersed in a liquid vehicle system therefor including a solvent and a synthetic water-insoluble film forming resin component for providing said film for corrosion resistance, said vehicle having reactive components susceptible to chemical reaction and combination with said pigment, a finely divided alloy material dispersed in said compositions including a first metal having an oxidation potential greater than that of said pigment and a second metal having an oxidation potential less than that of said pigment, said alloy being present in an amount of about 0.5% to 6% by weight of said liquid composition, said alloy operating to increase the electrical conductivity of said protective coating during welding and increasing the corrosion protection provided by said electroconductive pigment, the ratio of said resin to said pigment in the liquid coating composition falling approximately within the range of 1:1 to 1:20, said pigment being present in an amount of about 75 to 97% of the total solids in the dried coating and being present in the liquid composition as the elemental metal and uncombined and unreacted with the other components of said composition, a thixotropic agent present in said composition maintaining said pigment and said alloy evenly and uniformly distributed throughout said composition in a substantially uniform manner, contacting at least a portion of one article with another article of metal to be selected, passing a welding current through the two articles to be Welded at the coating thereon at the spots selected for welding, and fusing said article to the coating thereon together in the area of each spot weld.

References Cited in the file of this patent UNITED STATES PATENTS 1,770,540 Lunn July 15, 1930 2,243,832 Bohn June 3, 1941 2,452,805 Sussembach Nov. 2, 1948 2,726,308 Cinamon Dec. 6, 1955 2,970,204 Picen et al. Jan. 31, 1961 FOREIGN PATENTS 825,910 France Dec. 16, 1937 

1. A LIQUID COATING COMPOSITION FOR APPLICATION TO A METAL SUBSTRATE HAVING A GIVEN OXIDATION POTENTIAL FOR PROVIDING, WHEN DRIED ON SAID SUBSTRATE, A CORROSION RESISTANT AND ELECTROCONDUCTIVE WELDABLE COATING THEREON, COMPRISING A FINELY DIVIDED SOLID INORGANIC WATER-INSOLUBLE METALLIC ELECTROCONDUCTIVE PIGMENT HAVING AN OXIDATION POTENTIAL GREATER THAN THE OXIDATION POTENTIAL OF SAID SUBSTRATE, SAID PIGMENT BEING UNIFORMLY DISPERSED IN A LIQUID BINDER AND CARRIER THEREFOR INCLUDING A SOLVENT AND A SYNTHETIC FILM FORMING RESINOUS COMPONENT FOR ADHERING SAID PIGMENT TO SAID SUBSTRATE, A FINELY DIVIDED METALLIC ALLOY DISPERSED IN SAID COATING COMPOSITION INCLUDING A FIRST METAL HAVING OXIDATION POTENTIAL GREATER THAN THAT OF SAID PIGMENT AND A SECOND METAL HAVING AN OXIDATION POTENTIAL LESS THAN THAT OF SAID PIGMENT, SAID SECOND METAL ACTING AS AN AGENT EFFECTING CONTROLLED RELEASE OF ELECTRONS FROM SAID FIRST METAL FOR INCREASING THE CORROSION PROTECTION PROVIDED BY SAID PIGMENT, SAID ELECTROCONDUCTIVE PIGMENT BEING PRESENT IN AN AMOUNT OF 75% TO 97% OF THE TOTAL SOLIDS IN THE DRIED COATING, AND THE RATIO OF RESIN TO PIGMENT IN SAID LIQUID COATING COMPOSITION FALLING APPROXIMATELY WITHIN THE RANGE OF 1:1 TO 1:20. 